1. Field of the Invention
The present invention relates to a method and apparatus for applying a front face coating to ink jet printheads or ink jet printhead dies and, more particularly, to a method and apparatus for applying a longlasting hydrophobic coating to the front face of ink jet printer printheads or dies.
2. Description of the Related Art
In ink jet printing, a printhead is provided, the printhead having at least one ink-filled channel for communication with an ink supply chamber at one end of the ink-filled channel. An opposite end of the inkfilled channel has a nozzle opening from which droplets of ink are ejected onto a recording medium. In accordance with the ink droplet ejection, the printhead forms an image on the recording medium.
The ink droplets are formed as an ink meniscus at each nozzle opening experiences the momentum of the expanding bubble formed in the respective channel as a direct result of the voltage pulse applied across the heater in the channel. After a droplet is ejected, additional ink surges to the nozzle opening to re-form the meniscus.
The direction of the ink jet determines the accuracy of placement of the droplet which, in turn, determines the quality of printing performed by the printer. Accordingly, precise jet directionality is an important characteristic of a high quality printhead. Precise jet directionality ensures that ink droplets will be placed precisely where desired on the printed document. Poor jet directionality results in the generation of deformed characters and visually objectionable banding in half-tone pictorial images, particularly so with the newer generation of ink jet printers which enable printing at at least 300 dots per inch. In these high resolution printers the improved print quality demanded by the customer can only be provided if drop directionality were maintained over the entire useful life of the printhead.
Currently available ink jet printers provide accurate placement of ink droplets on a page for only a very limited period of time. The current printers do not maintain high print quality by maintaining the directionality of the ink jet throughout the entire printing lifetime of the printhead.
A major source of jet misdirection is associated with wetting of the front face of the printhead containing at least one nozzle opening. One factor which adversely affects jet directionality accuracy is the interaction of ink previously accumulated on the front face of the print head with the exiting droplets. This accumulation is a direct consequence of the forces of surface tension between the ink and the polar front face of the printhead, the accumulation becoming progressively severe with aging due to oxidation of the front face of the printhead. Ink may accumulate on the printhead front face due to either overflow during the refill surge of ink or the splatter of small droplets resulting from the process of ejecting droplets from the printhead or both. When ink accumulated on the front face of the print head makes contact with the ink meniscus at the nozzle orifice, the meniscus distorts, resulting in an imbalance of forces acting on the ejected droplet. This distortion leads to ink jet misdirection. This wetting phenomenon becomes more troublesome after extensive use of the printhead as the front face either oxidizes or becomes covered with dried ink film. As a result, gradual deterioration of the generated image quality occurs. One way of avoiding these problems is to control the wetting characteristics of the printhead front face so that no accumulation of ink occurs on the front face even after extensive printing. Thus, in order to provide accurate directionality for the exiting jet of ink, wetting of the front face of the printhead ought to be suppressed. This can be achieved by rendering the print head front face hydrophobic.
U.S. patent application Ser. No. 07/874,865, filed on Apr. 28, 1992 now U.S. Pat. No. 5,128,381, the disclosure of which is incorporated by reference herein, discloses a novel hydrophobic coating for the front face of a printhead in an ink jet printer. The coating controls the wetting characteristics of the front face to prevent ink accumulation on the front face. The coating comprises an epoxy adhesive (i.e., resin+curing agent) doped with a silicone rubber compound. The coating can be provided in the form of a 24% solution of the epoxy adhesive (resin+curing agent) and a 30:70 mixture of xylene and methyl iso-butyl ketone by weight doped with 1% by weight of the silicone rubber compound. The proportions of the components of the epoxy mix and the concentration of the epoxy mix in the solvent can be varied to suit a particular method of application. The coating enables the directionality of an ink jet to be maintained for the printing lifetime of the printer.
The application of such a long-lasting hydrophobic coating to the front face of an ink jet print head is extremely crucial to the maintenance of good print quality over the life of the ink jet printer.